The present invention relates to narrow-band excimer laser apparatus using a diffraction grating. More particularly, the present invention relates to a narrow-band excimer laser apparatus designed to suppress the spread of the line width of generated light due to wavefront distortion induced by a diffraction grating used as a component of a bandwidth-narrowing optical system.
Vacuum ultraviolet rays of wavelength around 193.3 nm generated from excimer laser apparatus, particularly ArF excimer laser apparatus, attract attention as being a next-generation laser light source for semiconductor lithography. As optical materials usable for exposure apparatus in the vacuum ultraviolet region, however, there are only synthetic quartz and fluorite, and it is extremely difficult to achromatize the optical system of the exposure apparatus. Accordingly, it is demanded that light generated from the excimer laser apparatus have an extremely narrow bandwidth (line width) of 0.5 pm or less.
Optical systems practically usable for narrowing the band of oscillation wavelengths in excimer laser apparatus include an etalon and a Littrow mounting reflection type diffraction grating. The former requires a high degree of surface accuracy and coating. In the present state of the art, only synthetic quartz can be used to obtain a high degree of accuracy. Synthetic quarts suffers from the problem that when the laser light intensity increases, synthetic quartz cannot be used because it suffers an increase in refractive index (compaction) and a reduction in transmittance. In addition, the line width widens undesirably when the coating becomes deteriorated. In contrast, in the case of the Littrow mounting reflection type diffraction grating, the line width is determined by the diameter of a beam incident on the reflection type diffraction grating. Therefore, the reflection type diffraction grating is used in combination with a beam diameter-enlarging optical system comprising a deviation prism or some deviation prisms. In this case, even when the prism becomes deteriorated, the beam diameter magnifying power does not change, and even when the reflecting surface of the reflection type diffraction grating becomes deteriorated, the groove pitch does not change. Therefore, there is no influence on the line width.
However, it has heretofore been impossible to narrow the bandwidth (line width) down to the desired levels by using a bandwidth-narrowing optical system including a beam diameter-enlarging prism or some beam diameter-enlarging prisms and a Littrow mounting reflection type diffraction grating as stated above; the narrowest possible bandwidth (line width) obtained with the conventional bandwidth-narrowing optical system is of the order of 0.6 pm. The reason for this is that wavefront distortion (i.e. the depth from the crest to the trough of the wavefront) induced by the diffraction grating is xcex/5 or more (a measured value for Hexe2x80x94Ne laser light), whereas wavefront distortion induced by an optical element (a prism, an output mirror, or a window) other than the diffraction grating is xcex/10 or less (a measured value for Hexe2x80x94Ne laser light).
In theory, wavelength resolution becomes higher as the angle of incidence on the reflection type diffraction grating is increased by increasing the blaze angle of the diffraction grating. However, if the blaze angle is increased in excess of 76xc2x0 to obtain a higher resolution, the surface finishing accuracy required for the grating grooves, on which incident light impinges normally in Littrow mounting, becomes higher. Actually, it is difficult to produce a reflection type diffraction grating of minimal variations in surface finish. Therefore, wavefront distortion due to the variations increases, making it impossible to narrow the bandwidth down to the order of 0.6 pm or less.
The present invention was made in view of the above-described problems with the prior art. Accordingly, an object of the present invention is to provide an excimer laser apparatus using a bandwidth-narrowing optical system including a beam diameter-enlarging system and a Littrow mounting reflection type diffraction grating. The excimer laser apparatus is made suitable for use as a laser light source for semiconductor lithography or the like by surmounting the limit to bandwidth narrowing due to wavefront distortion induced by the reflection type diffraction grating.
To attain the above-described object, the present invention provides a narrow-band excimer laser apparatus having a bandwidth-narrowing optical system including a Littrow mounting reflection type diffraction grating. The bandwidth-narrowing optical system further includes a beam diameter-enlarging optical system and a slit that are placed on the entrance side of the reflection type diffraction grating.
In the narrow-band excimer laser apparatus, diffracted wavefront distortion (a measured value for Hexe2x80x94Ne laser light) within the laser irradiation area of the reflection type diffraction grating in Littrow mounting is not more than xcex/10, where xcex is a measuring wavelength.
In addition, the present invention provides a narrow-band excimer laser apparatus having a bandwidth-narrowing optical system including a Littrow mounting reflection type diffraction grating. The bandwidth-narrowing optical system further includes a beam diameter-enlarging optical system and a slit that are placed on the entrance side of the reflection type diffraction grating.
In the narrow-band excimer laser apparatus, a light-blocking device is placed between the reflection type diffraction grating and the slit to prevent laser light from being applied to a portion of the laser irradiation area of the reflection type diffraction grating at which diffracted wavefront distortion (a measured value for Hexe2x80x94Ne laser light) in Littrow mounting is more than xcex/10, where xcex is a measuring wavelength.
It is desirable in the above-described narrow-band excimer laser apparatus that the number of grooves of the reflection type diffraction grating should be in the range of from 80 to 150 per millimeter.
Furthermore, it is desirable that the blaze angle of the reflection type diffraction grating should be not less than 76xc2x0.
In the present invention, diffracted wavefront distortion within the laser irradiation area of the reflection type diffraction grating in Littrow mounting is not more than xcex/10. Alternatively, a light-blocking device is placed between the reflection type diffraction grating and the slit to prevent laser light from being applied to a portion of the laser irradiation area of the reflection type diffraction grating at which diffracted wavefront distortion is more than xcex/10. Accordingly, the line width of generated laser light can be made extremely narrow, i.e. 0.5 pm or less. Thus, it is possible to realize a narrow-band excimer laser apparatus satisfactorily usable as a laser light source for semiconductor lithography or the like.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.